I. Field
The subject technology relates generally to communications systems and methods, and more particularly to systems and methods that perform enhanced time synchronization and channel estimation in accordance with wireless networks.
II. Background
Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. These channels are sometimes called subbands or subcarriers. The technology was first conceived during research into minimizing interference among channels near each other in frequency. In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Generally, priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream. Less importance is placed on perfecting individual channels.
In one area, OFDM has also been used in European digital audio broadcast services. The technology lends itself to digital television, and is being considered as a method of obtaining high-speed digital data transmission over conventional telephone lines. It is also used in wireless local area networks. Orthogonal Frequency Division Multiplexing can be considered an FDM modulation technique for transmitting large amounts of digital data over a radio wave where OFDM operates by splitting a radio signal into multiple smaller sub-signals or sub-carriers that are then transmitted simultaneously at different frequencies to the receiver. One advantage of OFDM technology is that it reduces the amount of crosstalk in signal transmissions where current specifications such as 802.11a WLAN, 802.16 and WiMAX technologies employ various OFDM aspects.
In some systems deploying OFDM technology, transmissions are intended for many users simultaneously. One such example is a broadcast or multicast system. Further, if different users can choose between different portions of the same transmission, the data in each transmission is typically time division multiplexed (TDM). It is often the case that the data intended for transmission is organized into fixed structures such as frames or superframes. Different users can then choose to receive different portions of a superframe at any given time. In order to assist the multitude of users with synchronization to the timing and frequency of the broadcast signal, time division multiplexed (TDM) pilot symbols are sometimes inserted at the beginning of each superframe. In one such case, each superframe begins with a header consisting, among other things, of two TDM pilots, called TDM pilot 1 and TDM pilot 2. These symbols are used by the system to achieve initial frame synchronization, also called initial acquisition.
In order to further assist with time and/or frequency synchronization during a superframe, also called time or frequency tracking, additional pilot symbols may be used. Time and frequency tracking may be achieved using the frequency division multiplexed (FDM) pilots, which may be embedded in each transmitted data OFDM symbol. For instance, if each OFDM symbol consists of N subcarriers, N-P of them can be used for data transmission and P of them can be assigned to FDM pilots. These P FDM pilots are sometimes uniformly spread over the N subcarriers, so that each two pilots are separated by N/P-1 data subcarriers. Such uniform subsets of subcarriers within an OFDM symbol are called interlaces.
Time domain channel estimates are used for time tracking during a superframe. Time domain channel estimates are obtained from FDM pilots, embedded in data OFDM symbols. The FDM pilots can be always placed on the same interlace, or they can occupy different interlaces in different OFDM symbols. The subset of subcarriers with indices i+8k is sometimes called the ith interlace. In this instance, N/P=8. In one case, the FDM pilots can be placed on interlace 2 during one OFDM symbol, on interlace 6 during the following symbol, then back on interlace 2 and so forth. This is called (2,6) staggering pattern. In other instances, the pilot staggering pattern can be more complicated, so that the occupied interlaces describe the pattern (0,3,6,1,4,7,2,5). This is sometimes called the (0,3,6) staggering pattern. Different staggering patterns make it possible for the receiver to obtain channel estimates longer than P time-domain taps. For example, (2,6) staggering pattern can be used at the receiver to obtain channel estimates of length 2P, while (0,3,6) staggering pattern can lead to channel estimates of length 3P. This is achieved by combining the channel observations of length P from consecutive OFDM symbols into a longer channel estimate in a unit called the time filtering unit. Longer channel estimates in general may lead to more robust timing synchronization algorithms.
Some broadcast systems are intended for different types of transmission simultaneously. For example, some of the broadcast data may be intended for any potential user within the national network, and such data is called national or wide area-area content. Other data symbols transmitted on the network may be intended only for users currently residing in a specific, local portion of the network. Such data is called local-area content. The data OFDM symbols, belonging to different contents may be time division multiplexed within each frame in a superframe. For example, certain portions of each frame within a superframe may be reserved for wide area content and the other portions for local area content. In such cases, the data and pilots intended for different contents can be scrambled using different methods. Moreover, the set of transmitters that are simultaneously broadcasting the wide area and the local area content within a superframe can be different. It is therefore quite common that the time domain channel estimates, as well as channel observations, associated with wide area content and those associated with local area content can be quite different. In these scenarios, special strategy needs to be deployed for channel estimation on OFDM symbols grouped near the boundary between the wide area and local area content. A special waveform needs to be transmitted to aid time tracking and channel estimation before and after the content boundary.